A brushless direct current (BLDC) motor is generally divided into a core type BLDC motor (or radial gap type BLDC motor) having a cup (cylindrical) structure and a coreless type BLDC motor (or axial gap type BLDC motor) depending on whether or not a stator core is present.
The core type BLDC motor is classified into an inner ring-shaped magnet type BLDC motor including a cylindrical stator having a coil wound therearound in order to have an entire ring type magnet structure on a plurality of protrusions (teeth) formed on an inner peripheral portion thereof and a rotor configured of a cylindrical permanent ring-shaped magnet and an outer ring-shaped magnet type BLDC motor including a stator having a coil wound around a plurality of protrusions (teeth) formed on an outer peripheral part thereof in a vertical direction and a rotor configured of a cylindrical permanent ring-shaped magnet multi-pole magnetized at an outer portion thereof.
Since the core type BLDC motor includes a magnetic circuit having a symmetrical structure in a radial direction based on a shaft, the core type BLDC motor has low axial vibration noise, is appropriate for low speed rotation, and has a very small portion occupied by an air gap in a direction of a magnetic path. Therefore, even though a ring-shaped magnet having low performance is used or a volume of ring-shaped magnet is decreased, a high magnetic flux density may be obtained, such that a torque is large and efficiency is high.
Meanwhile, the ring-shaped magnet installed in the BLDC motor is subjected to a magnetizing process in order to drive the motor and generate a frequency.
FIG. 1 is a schematic view of a device for magnetizing a ring-shaped magnet for a BLDC motor according to the relate art.
As illustrated in FIG. 1, the device for magnetizing a ring-shaped magnet for a BLDC motor according to the relate art is configured to include a rotor 10 including a rotor core 12 having a shaft 11 inserted thereinto and a ring-shaped magnet 20 installed on an outer peripheral surface of the rotor core; and a plurality of magnetizing yokes 30 installed so as to face an outer peripheral surface of the ring-shaped magnet.
Here, a magnetizing voltage is applied to the magnetizing yokes 30, such that a magnetic flux is generated through a magnetizing iron core, and even-numbered unit magnets 21 are magnetized in the ring-shaped magnet 20 by the magnetic flux.
The even-numbered unit magnets 21 are coupled to each other, such that one of them may have magnetism of an S pole and the other of them may have magnetism of an N pole.
FIG. 2 is a graph illustrating magnetic flux densities of unit magnets formed by magnetizing the ring-shaped magnet for a BLDC motor according to the relate art depending on an electrical degree.
A portion illustrated by electrical degrees from 0 degree to 180 degrees at the left of the graph of FIG. 2 is a graph illustrating a magnetic flux density of a unit magnet having magnetism of an N pole depending on an electrical degree, the electrical degrees from 0 degree to 180 degrees in FIG. 2 mean portions from one end of the unit magnet having the magnetism of the N pole to the other end thereof, and electrical degrees from 180 degrees to 360 degrees mean portions from one end of a unit magnet having magnetism of an S pole to the other end thereof.
Referring to FIG. 2, it may be confirmed that ‘dead zones’ in which a magnetic flux density is rapidly changed are generated in one end and the other end of the unit magnet having the magnetism of the N pole and one end and the other end of the unit magnet having the magnetism of the S pole.
Therefore, according to the related art, there is a problem that a cogging phenomenon and a torque ripple phenomenon, which are rotation hunting phenomena of the ring-shaped magnet due to a rapid change in a magnetic resistance caused by the ‘dead zones’ of the even-numbered unit magnets configuring the ring-shaped magnet, occurs.
Therefore, the development of various devices for magnetizing a ring-shaped magnet for a BLDC motor for solving the above-mentioned problem has been demanded.
As a technology associated with this, a device for magnetizing a motor magnet has been suggested in Korean Patent Laid-Open Publication No. 2000-0044134.